Transglutaminases belong to a family of enzymes that play important roles in diverse biological functions by selectively cross-linking proteins. They catalyze formation of ε-(γ-glutamyl)-lysine cross-links between proteins, and may also incorporate polyamines into suitable protein substrates. This covalent isopeptide cross-link is stable and resistant to proteolysis, thereby increasing the resistance of tissue to chemical, enzymatic, and mechanical disruption. Among the members of this family are plasma transglutaminase, factor XIIIa, which stabilizes fibrin clots; keratinocyte transglutaminase and epidermal transglutaminase, which cross-link proteins on the outer surface of squamous epithelia; and tissue transglutaminase, which cross-links fibronectin in the extracellular matrix of organs such as brain, liver and the intestine.
Transglutaminase 2 (TG2, also known as tissue transglutaminase), a calcium-dependent member of the transglutaminase family, is reported to have extracellular as well as intracellular functions. Outside the cell TG2 plays a crucial role in shaping the extracellular matrix by cross-linking fibronectin and related proteins. TG2 also promotes cell adhesion and motility by forming non-covalent complexes with other key proteins such as integrins and fibronectin. Intracellular TG2 loses enzyme activity when bound to GTP, but functions as a G-protein in the phospholipase C signal transduction cascade. Human TG2 is a structurally and mechanistically complex protein. Its catalytic mechanism is similar to that employed by cysteine proteases, involving a catalytic triad of cysteine, histidine, and aspartate. The cysteine thiol group reacts with a glutamine sidechain of a protein substrate to form a reactive thioester intermediate, from which the acyl group is transferred to another amine substrate.
Several members of the transglutaminase family have been linked to disease, including tissue transglutaminase (TG2), and the skin transglutaminases, TG1 and TG3. TG2 is a cytoplasmic enzyme present in many cells, including those in the blood vessel wall. Aberrant TG2 activity is believed to play a role in neurological disorders such as Alzheimer's, Parkinson's and Huntington's disease (see, for example, Kim et al. (2002) Neurochem. Int. 40:85-103; Karpuj et al. (2002) Nature Med. 8, 143-149). In Celiac Sprue, where TG2 is the predominant autoantigen, its pivotal role in unmasking antigenic epitopes by site specific deamidation of gluten peptides is well established. Expression of TG1 and TG2 have been correlated with various types of malignancies (see Zhang et al. (2003) Glia 42:194-208; and Martinet et al. (2993) Am. J. Respir. Cell. Mol. Biol. 28, 428-435), including glioblastomas, lung and breast cancers, suggesting an important role for TG2 in tumor proliferation and survival. Taken together, the above findings make a strong case for suitable small molecule TG2 inhibitors as experimental therapeutic agents. The medicinal attractiveness of this protein target is underscored by the observation that TG2 knockout mice are normal, lacking developmental, physiological or reproductive defects.
Although a number of TG2 inhibitors have been used in biological studies over the past two decades, many of these compounds (e.g. monodansyl cadaverine) contain primary amines in addition to potential inhibitory motifs, and it remains unclear whether the observed effects are due to excess competing amines or by blockage of TG2 substrate turnover. A few studies have utilized a suicide inhibitor, L682777, which inhibits human TG2 (Lorand et al. (1998). Exp Eye Res. 66:531-6). However, L682777 was designed as a specific inhibitor of Factor XIIIa, and is therefore unsuitable for evaluating TG2 biology in vivo. More recently, mechanism-based active-site inhibitors of guinea pig and human (Hausch et al. (2003) Chem Biol 10, 225-231; Choi et al. (2005)Chem. Biol. 12, 469-475) TG2 have been reported.
In view of the serious and widespread nature of Celiac Sprue and the difficulty of removing gluten from the diet, better methods of treatment are of great interest. In particular, there is a need for treatment methods that allow the Celiac Sprue individual to eat gluten-containing foodstuffs without ill effect or at least to tolerate such foodstuffs in small or moderate quantities without inducing relapse. The present invention meets this need for better therapies for Celiac Sprue by providing new drugs and methods and formulations of new and existing drugs to treat Celiac Sprue. International Patent Application US03/04743, herein specifically incorporated by reference, discloses aspects of gluten protease stability and immunogenicity.
tTGase has also been implicated in certain cancers. Neuro-oncological diseases including malignant neoplasms such as glioblastomas and melanomas metastatic to the brain are notoriously resistant to standard radiation and chemotherapy treatment. Current treatment strategies generally fail to achieve long-term survival. Similarly certain benign CNS tumors such as meningiomas are resistant to chemotherapy and radiation. Current treatment strategies with these tumors typically require major surgical resections or treatment with radiation in an attempt to control growth of recurrent or non-resectable tumors. Meningiomas are generally resistant to radiation-induced cell death and to chemotherapy. The mechanisms responsible for the failure of these brain tumors to respond to chemotherapy and radiation are not known. Therefore, identification of agents that augment sensitivity to chemotherapy and radiation therapy is important for improving treatment strategies in patients with these and other refractory cancers.